ReadonlycolorThe mark's resolved style colour.
ReadonlyidThe series identity — the layer's id prop. The selection / dedup /
controlled-echo key; stable across data updates (unlike key).
ReadonlykeyThe clicked sample's key as epoch ms (its event's begin) — click
provenance, informational. NOT the selection identity (that is id).
ReadonlylabelDisplay label (as ?? column ?? id) — labels the selection in a readout.
Optional ReadonlymarkAn optional stable per-mark identity within the layer — a category's
column name on the categorical axis, where every bar shares the layer's
id but each column needs its own stable handle. When present, the
highlight match + controlled selected echo key on (id, mark) instead of
the sample key, so a pinned selection survives a column reorder / data
update (the slot index is not stable; the column name is). undefined for
marks whose sample key is already their identity (a time / value bar).
ReadonlyvalueThe clicked sample's value (the plotted column) — provenance.
One selection — what RowLayer.hitTest returns and
onSelectreports. Selection identity is the seriesid, not the sample:key/valueare click provenance (the nearest sample under the pointer, informational); equality, dedup, and the controlled echo all key onid. Becauseidis a stable series identity — distinct from theastheme role, which can repeat — a selection survives a streaming data update where a samplekeywould go stale. Only layers that carry anidare selectable (see RowLayer.hitTest).